Essential Intervals in Music Theory

Why This Matters

Intervals aren't just abstract distances between notes—they're the emotional DNA of music. Every melody you've ever hummed, every chord that gave you chills, every moment of tension before a resolution: it all comes down to intervals. When you're tested on music theory, you're being assessed on your ability to hear these relationships, identify them by ear and on paper, and explain why certain combinations sound stable while others create tension that demands resolution.

The key to mastering intervals is understanding that they fall into distinct categories based on their acoustic properties and emotional character. Perfect intervals provide stability and openness. Major intervals tend toward brightness. Minor intervals lean melancholic. Dissonant intervals create tension. Don't just memorize the half-step counts—know what each interval does in a musical context and why composers reach for specific intervals to achieve specific effects.

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Perfect Intervals: The Stable Foundation

Perfect intervals—the unison, fourth, fifth, and octave—share a unique acoustic property: their frequency ratios are simple whole numbers, creating maximum consonance. These intervals have been considered "perfect" since ancient Greek music theory because they sound stable, open, and complete.

Perfect Unison

• Two identical pitches sounding together—the simplest possible interval, representing complete unity
• Notated as P1 and spans zero half steps; often used to reinforce melodic lines in orchestration
• Creates maximum stability because there's no harmonic tension whatsoever—useful for emphasizing important notes

Perfect Fourth

• Five half steps between pitches—sounds open and somewhat hollow
• Notated as P4 and considered consonant in most contexts, though it can function as a dissonance in counterpoint
• Context-dependent stability—stable in parallel motion, but creates tension when placed above the bass note in classical harmony

Perfect Fifth

• Seven half steps apart—one of the most consonant intervals in Western music
• Notated as P5 and forms the foundation of power chords, open tunings, and the overtone series
• Acoustically pure because of its 3:2 frequency ratio; your ear recognizes this as fundamentally stable

Perfect Octave

• Twelve half steps—the interval where pitches share the same letter name
• Notated as P8 and perceived as the same note at a different register
• Fundamental to pitch organization—defines the repeating pattern of our musical alphabet (A through G)

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Consonant Intervals: Major and Minor Thirds and Sixths

These intervals define whether music sounds major (bright, happy) or minor (dark, sad). The third is particularly crucial because it determines chord quality—change the third, and you transform the entire emotional character.

Minor Third

• Three half steps—the defining interval of minor chords and keys
• Notated as m3 and evokes melancholy, introspection, or tension
• Appears in minor triads as the distance from root to third; instantly recognizable in songs like "Greensleeves"

Major Third

• Four half steps—the bright, stable interval that defines major tonality
• Notated as M3 and creates the happy, resolved quality of major chords
• Foundation of major triads—the first two distinct notes of "Here Comes the Bride" outline a major third

Minor Sixth

• Eight half steps—a wider interval with a slightly dark, unresolved quality
• Notated as m6 and commonly appears in jazz voicings and romantic-era harmony
• Inverts to a major third—understanding inversions helps you recognize this interval in different contexts

Major Sixth

• Nine half steps—warm, consonant, and often used for melodic leaps
• Notated as M6 and creates a sense of yearning or reaching upward
• Opens "My Bonnie Lies Over the Ocean"—a classic example for ear training

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Seconds and Sevenths: Tension and Color

These intervals sit close together (seconds) or nearly span an octave (sevenths), creating inherent instability that demands resolution. They're essential for creating movement in music—without tension, there's no release.

Minor Second

• One half step—the smallest interval in Western music
• Notated as m2 and sounds sharply dissonant, like two adjacent piano keys
• Creates maximum tension—think of the "Jaws" theme; that creeping dread is built on minor seconds

Major Second

• Two half steps—the standard "whole step" of scales
• Notated as M2 and sounds mildly dissonant but melodically smooth
• Building block of scales—most scale passages move by major seconds; it's the "default" melodic motion

Minor Seventh

• Ten half steps—tense but not harsh, with a bluesy character
• Notated as m7 and defines the sound of dominant seventh chords
• Essential in jazz and blues—creates the "pull" that makes V7 chords want to resolve to I

Major Seventh

• Eleven half steps—just one half step shy of an octave
• Notated as M7 and sounds lush, complex, and sophisticated
• Signature of jazz harmony—major seventh chords (like Cmaj7) have that dreamy, unresolved quality

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The Tritone: Maximum Instability

The tritone stands alone as the most dissonant interval in tonal music. It divides the octave exactly in half, creating an unsettling symmetry that the ear struggles to place.

Tritone (Augmented Fourth/Diminished Fifth)

• Six half steps—exactly halfway through the octave
• Notated as A4 or d5 depending on spelling; historically called "diabolus in musica" (the devil in music)
• Drives harmonic resolution—the tritone in a dominant seventh chord creates the tension that pulls toward the tonic

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Quick Reference Table

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Self-Check Questions

1. Which two intervals define whether a chord sounds major or minor, and how many half steps apart are they from each other?

2. Compare the Perfect Fourth and Perfect Fifth: both are "perfect" intervals, so why might a P4 above the bass create tension in classical harmony while a P5 does not?

3. A dominant seventh chord contains a tritone. Between which two scale degrees does this tritone occur, and why does it create such a strong pull toward resolution?

4. You hear an interval that sounds "sad" and spans three half steps. What is it called, and what would you need to change to make it sound "happy"?

5. Rank these intervals from most consonant to most dissonant: Major Third, Minor Second, Perfect Fifth, Tritone, Major Seventh. Explain your reasoning using acoustic principles.